The invention relates to intraluminal endovascular stenting, and in particular, to a low profile stent.
Endovascular stenting is particularly useful for arteries which are blocked or narrowed and is an alternative to surgical procedures that intend to bypass the occlusion. The procedure involves inserting a prosthesis into a body lumen and expanding it to prevent collapse of a vessel wall. While stenting has most commonly been used adjunctively, following an intervention such as angioplasty or atherectomy, there is increasing interest in primary, or direct stent placement.
Percutaneous transluminal angioplasty (PTCA) is used to open coronary arteries which have been occluded by a build-up of cholesterol fats or atherosclerotic plaque. Typically, a guide catheter is inserted into a major artery in the groin and is passed to the heart, providing a conduit to the ostia of the coronary arteries from outside the body. A balloon catheter and guidewire are advanced through the guiding catheter and steered through the coronary vasculature to the site of therapy. The balloon at the distal end of the catheter is inflated, causing the site of the stenosis to widen. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten re-closure of the dilated vessel or even perforations in the vessel wall. Implantation of a stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. Reducing the possibility of restenosis after angioplasty reduces the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
A stent is typically a cylindrically shaped device formed from wire(s) or a tube and is intended to act as a permanent prosthesis. A stent is deployed in a body lumen from a radially compressed configuration into a radially expanded configuration which allows it to contact and support a body lumen. The stent can be made to be radially self-expanding or expandable by the use of an expansion device. The self-expanding stent is made from a resilient springy material while the expandable stent is made from a material which is plastically deformable. A plastically deformable stent can be implanted during an angioplasty procedure by using a balloon catheter bearing a crimped, or compressed stent which has been loaded onto the balloon. The stent radially expands as the balloon is inflated, forcing the stent into contact with the body lumen, thereby forming a supporting relationship with the vessel wall. Deployment is effected after the stent has been introduced percutaneously, transported transluminally and positioned at a desired location by means of the balloon catheter.
A balloon of appropriate size and pressure is first used to open the lesion. The process can be repeated with a stent loaded onto a balloon. Direct stenting involves simultaneously performing angioplasty and stent implantation using a stent mounted on a dilatation balloon. The stent remains as a permanent scaffold after the balloon is withdrawn. A balloon capable of withstanding relatively high inflation pressures may be preferable for stent deployment because the stent must be forced against the artery""s interior wall so that it will fully expand, thereby precluding the ends of the stent from hanging down into the channel, encouraging the formation of thrombus.
In adjunctive stenting, a stent delivery system with a small diameter profile is not required because the narrowing has already been enlarged by the preceding device. However, in direct stenting, the stent and delivery balloon catheter need to be inserted into a stenosis that has not been previously dilated. Thus, for direct stenting to be applicable to many patients, the stent and delivery system must have a very low profile. The primary advantage of direct stenting is the procedural efficiency gained by eliminating a primary angioplasty step. The resulting procedure can be shorter and less expensive.
Primary angioplasty followed by stent placement typically requires a catheter exchange, which is usually performed over a guidewire. Given the prevalence of this staged procedure, the most commonly used balloon catheters have been over-the-wire types, having either a full length guidewire lumen or a short, distal guidewire lumen as found in rapid exchange catheters. Fixed wire, or xe2x80x9cballoon-on-a-wirexe2x80x9d type balloon catheters have been seldom used for primary angioplasty in stenting procedures, and these catheters have not been used to deliver stents at all. With their small size and wire-like trackability, fixed wire catheters are able to provide relatively quick and simple balloon placement and access to lesions that cannot be reached with other types of catheters. The small size of fixed wire catheters also permits their use through very small guiding catheters. However, these balloon catheters lack the ability to maintain guidewire position across a lesion for exchange purposes and they may encounter problems re-crossing a dilated area. Another reason that fixed wire balloon catheters have not been used for stent delivery is that the very small deflated profile of the balloon on such a catheter may be too small to securely carry a compressed stent of conventional design.
Previous structures used as stents or intraluminal vascular grafts have included coiled stainless steel springs, helically wound spring coils made from a shape memory alloy, expanding metal stents formed in a zig-zag pattern, and diamond shaped, rectangular shaped, and other mesh and non-mesh designs. Exemplary stent devices are disclosed in U.S. Pat. No. 5,776,161 issued to Globerman, U.S. Pat. No. 5,449,373 issued to Pinchasik et al, U.S. Pat. No. 5,643,312 issued to Fischell et al and U.S. Pat. No. 5,421,955 issued to Lau et al.
Problems to be overcome in stent design include inadequate radial force to maintain stent expansion, inadequate scaffolding of tissue against the wall, predilatation longitudinal rigidity that negatively impacts on stent delivery, and shortening of the stent as a consequence of radial expansion. Predilatation longitudinal rigidity is a significant stent shortcoming that prevents the threading of the stent through long tortuous vessels and lesions. Shortening of the stent is also a problem, as it is important that the stent cover the entire lesion to minimize the risk of post-operative complications. Many of these problems result from the often conflicting goals of stent design. For example, to provide uniform support to the vessel wall, it is desirable to have a high degree of scaffolding in the stent when it is expanded to its nominal radial size. However, it is also desirable to have a small, relatively smooth delivered profile when the stent is mounted on the catheter to permit the stent and catheter to traverse small diameter lesions. The person skilled in the art will appreciate that, as a stent with a very small delivered profile expands radially, its structural elements become farther apart and create openings which reduce the amount of scaffolding available to support the vessel. A similar situation exists with respect to the conflicting goals of improved scaffolding and flexibility during catheter delivery since proper scaffolding will not be accomplished if there are too few supporting structural elements. However, a stent with too many structural elements may be difficult to crimp small enough to fit onto the balloon catheter such that the structural elements do not abut or interfere with each other during delivery through tortuous vessels. Also, in some stents, during plastic deformation of the stent (i.e. balloon expansion), the strain is concentrated at small zones. This limits the properties of the material that can be used as well as the radial force and the expansion rate.
Co-pending U.S. patent application Ser. No. 09/292,991, entitled Medical Device for Intraluminal Endovascular Stenting, addresses a number of these issues. The ""991 application discloses an expandable stent having a small initial diameter, flexibility along its longitudinal axis prior to expansion and minimization of rigid local strain on the stent material by the presence of rotation joints which have minimal strain during stent expansion. The stent of the ""991 application has substantially the same length before and after expansion and it is easy to deliver, being longitudinally flexible when constrained. However, the delivery of such a stent on very low profile, fixed wire delivery catheters requires additional improvements in the size of the minimal crimped diameter.
The stent of the present invention has a hollow, cylindrical body made with a plurality of rings. It is manufactured from tubing having a diameter between a minimal crimped diameter of the stent and a nominal size that is closely matched to the size of the vessel to be treated. In the manufactured form of the stent, each ring extends circumferentially around the cylindrical body and includes an undulating series of peaks and valleys. The undulating peaks and valleys of each ring are formed by opposing angled segments, which are joined to each other by substantially straight struts. Adjacent rings are joined together by a circumferentially disposed series of links, which are shaped and arranged to promote longitudinal flexibility as the stent is delivered by the catheter. The links also promote effective scaffolding after stent deployment and prevent shortening of the stent as it is expanded.
Each ring is provided with inflection points, which may be substantially centered on the struts that extend between adjacent peaks and valleys of the ring. At each inflection point, a portion of the ring extends in a generally circumferential direction for a short distance, producing an offset in the otherwise straight strut. A link is joined at one end to an inflection point on one ring and the link is also joined at a second end to a second inflection point on an adjacent ring. Preferably, when the stent device is unexpanded, each link includes at least two angled segments that are capable of deflecting to promote longitudinal flexing of the stent when it is subjected to bending forces, such as those encountered during delivery of the stent and catheter through a tortuous vascular anatomy. Also preferably, each inflection point has a length, measured circumferentially, which is at least as great as the width of the link to which it is attached. Preferably, the circumferential length of the inflection point is no more than about twice the width of the link to which it is attached. When the stent is crimped on the balloon catheter, the links can fit closely together in a nested arrangement with the undulations of the rings. When the stent is expanded, vessel scaffolding is promoted by this nested arrangement, which allows a large number of connecting links. The terms xe2x80x9cnestxe2x80x9d, xe2x80x9cnestedxe2x80x9d or nestingxe2x80x9d are used herein to mean that the elements are conformally arranged such they can be in very close proximity when the stent is crimped and loaded onto a catheter but without substantial contact that would affect the ability of the various elements to move in relation to each other as the stent and catheter are advanced through a tortuous body vessel. In some embodiments of the invention, only one link is connected to an inflection point. This makes the inflection point a xe2x80x9cdead endxe2x80x9d and permits some of the flexing forces which are not absorbed by the link itself to be absorbed by the rings to which it is attached. The links are arranged to provide flexibility, and the peaks and valleys of the rings are paired with each other in an in-phase relationship. Preferably, the rings are joined by multiple links, most preferably three or more links, and each ring has the same number of inflection points as the number of attached links. When a large number of connecting links are employed, the angles in the links are preferably of a complimentary shape to each other such that they will nest together when the stent is crimped for mounting onto the catheter.
Another aspect of the invention is the conformal nesting of ring and link components such that the stent can be readily crimped for loading onto a balloon or other expansion device on the catheter. A stent made according to the present invention may be made from a tube which is cut with lasers or other techniques that are well known to those skilled in the art. The initial pattern cut into the tube includes sufficient spacing between link and ring components such that the stent can be crimped onto a catheter without causing general abutment of the ring and link components with each other. During deployment of the stent catheter through tortuous coronary arteries, the pattern also permits longitudinal movement of the link components without disturbing the crimp of the ring components. The need for spacing between the components in the crimped condition must be balanced with the need to provide improved scaffolding of the vessel being treated. That is, a relatively abundant number of links provides improved scaffolding of the vessel but potentially interferes with the ability to crimp the stent onto the catheter.
In another aspect of the invention, the stent configuration has undulating peaks and valleys of adjacent rings paired with each other in an in-phase relationship. Such a configuration can allow interconnecting links to nest within the peaks and valleys of the rings by providing at least two angled segments in a central portion of the link.
The compressed stent of the present invention can be securely mounted onto a very low profile, fixed wire catheter by incorporating relatively sharp angled bends instead of gentle curves and by placing adjacent portions of links and rings close together to provide the spacing needed for more compact nesting of the ring and link components. A further aid in reducing the minimal crimp diameter of the stent is to make the rings and links as narrow as possibly permitted by other design constraints. Thus, in the present invention, large numbers of connecting links can be included within a stent design having a very low minimal crimp diameter.